In the production of modules containing integrated circuits, or surface-mountable components (SMD), usually the circuits and components are first produced on or in semiconductor wafers by means of microelectronic technologies and circuit arrangements are thus formed, contact terminals for electrical connection to an external contact device being applied on the surface of the semiconductor circuit arrangement. The semiconductor circuit arrangements or semiconductor chips are then singulated, connected to external contact devices and enclosed by a housing. Within this process, the semiconductor circuit arrangements are electrically characterized and mounted on a carrier and the electrical connection to external terminals is produced.
Contact terminals on the surface of semiconductor circuit arrangements are subjected to various ambient influences during the processing, the electrical characterization of the components and circuits contained and the subsequent production of the electrical connection to an external contact device. Thus, by way of example, as a result of needle card compressions during the measuring operation or as a result of the bonding, cracks may arise in the metallization of the contact terminal, the cracks promoting diffusion of alkali metal ions, e.g. Na ions, into the semiconductor circuit arrangement, specifically into the active areas, and thus leading for example to a shift in the threshold voltage of transistors.
The electrical connections between a semiconductor circuit arrangement and an external contact device can be implemented in various ways. One very widespread method is wire bonding using an Au or Al wire. In this case, a first end of the wire is fixed on the internal contact terminal on the surface of the semiconductor circuit arrangement, the wire is subsequently led from there to an external contact terminal on the surface of the external contact device and the second end of the wire is fixed on the external contact terminal.
Various methods which differ with regard to the materials used and the process parameters used, such as e.g. pressure and temperature, are customary in the case of wire bonding. Thermocompression bonding involves using an Au wire, the end of which is melted and pressed onto the contact terminal. Ultrasonic bonding involves fixing an Al wire, for example, on the contact terminal by means of a cold welding compression connection, that is to say at room temperature with application of ultrasound. A combination of both methods is also possible.
Producing the electrical connection by means of bonding entails some serious problems. During bonding on the surface of the semiconductor circuit arrangement, it is possible, primarily in the case of process parameters which lead to a great deformation of the bond (overbonding), for cracks to occur in the contact metallization, the cracks promoting diffusion of alkali metal ends, e.g. Na ions, into the active regions of the semiconductor circuit arrangement and thus leading to an alteration of the electrical properties of the semiconductor circuit arrangement. The bonding yield and the reliability of the bonding connections, e.g. the power cycling strength, are dependent on the process parameters chosen. Thus, a low pressure during the production of the bonding connection may increase the yield, but it simultaneously reduces the reliability, whereas although an increased pressure during the production of the bonding connection and thus an enlarged bearing area of the wire on the contact terminal increase the reliability, they nonetheless lead for example to the abovementioned problems with cracks in the metallization.
This problem is relevant particularly when using thick wires such as are employed in the bonding of power semiconductor modules on account of the high current-carrying capacity acquired.
Furthermore, corrosive molding compound materials may attack the bonding contact, which reduces the strength and reliability of the contact. These materials are part of the housing which encloses the semiconductor circuit arrangement and the external contact device. Furthermore, portions of the bonding contact are observed to be etched away from the contact terminal during subsequent processes on account of different chemical potentials.
In order to solve these problems, complicated optimization processes have been carried out hitherto for the electrical characterization by means of needle cards and for the bonding parameters, but although they have brought about an improvement, they do not constitute a final solution to the problem. Furthermore, the metallization on the surface of the semiconductor circuit arrangement has been embodied as a very thick layer or mechanically hardened by the targeted admixture of additional materials in order that process parameters which enable a high yield and reliability can be used during bonding. For mechanical stabilization of the bonding contact on the surface of the semiconductor circuit arrangement, which is necessary particularly in the case of bonding processes without great deformation of the bonding wire, and also protection against aggressive molding compound materials, a hard, high-temperature-resistant plastic is applied to the bonding contact. However, all these solution approaches lead to a significant increase in costs. There is currently no solution for the problem of the bonding contact being etched away on account of different chemical potentials.
U.S. Pat. No. 6,825,564 discloses a method in which an NiP layer was deposited in electroless fashion on a contact terminal made of Cu in order to prevent the oxidation of the Cu surface and thus to improve the bondability of the contact terminal.
U.S. Pat. No. 6,564,449 describes a method for reinforcing the contact terminal on the surface of the semiconductor circuit arrangement, in which an additional layer system made of Ti, Cr or TiW and Au or made of Ni and Au is deposited on the contact terminal in order to increase the reliability and reproducibility of the bonding contact in the case of reverse wire bonding using Au wire.
DE 26 50 348 A1 describes a connection arrangement in which a layer system comprising an Ni layer and an Au layer was applied to the pin-type electrical connecting conductors of a discrete electrical component and to the conductor track to be connected thereto on a printed circuit board, in order to improve the solderability of the contact terminals.
A use of Cu as material for the wire is envisaged in the future on account of the good electrical properties of Cu. This is not yet possible at the present time, however, since high forces are necessary when producing the contact between the Cu wire and the contact terminal, which, in the case of the present-day embodiment of the contact terminals on the surface of the semiconductor circuit arrangement, leads to damage in the contact terminal and the underlying regions of the semiconductor circuit arrangement.
For these and other reasons, there is a need for the present invention.